Direct current motor control circuit and wiper system using said circuit

ABSTRACT

The invention relates to a direct current motor control circuit. The invention also relates to the use thereof in a windscreen wiper system for vehicles. An MOS PET transistor with an anti-parallel diode (M 3 ) is serially mounted on the braking system (M 2 ) of the control circuit.

The present invention relates to a control circuit for controlling adirect current (DC) electric motor.

It also relates to the use of such a control circuit in a vehiclewindshield wiper system.

In the state of the art, it is known that a DC motor can be powered bymeans of a “half-bridge” control circuit in which a metal oxidesemiconductor (MOS) type transistor makes it possible to connect a powersupply terminal of the motor to the positive terminal of a vehiclebattery.

The other terminal of the DC motor is connected to the pack or to thenegative terminal of the DC power supply source.

In the state of the art, such a control circuit is generallysupplemented by a second MOS transistor that is connected between theground connected to the pack and that terminal of the electric motorwhich is connected to the above-mentioned MOS transistor.

The second MOS transistor is generally dimensioned to receive littlecurrent, and it is activated, i.e. caused to conduct, only while the DCmotor is in its braking phase.

For the remainder of the operating time, the second MOS transistor isput into its high-impedance state and sees no current, other than aleakage current, pass through its drain-source path.

Such a control circuit must be highly reliable.

Particularly during vehicle maintenance operations, reversing thepolarities applied to the control circuit of the electric motor can giverise to destruction of the circuit because, in order to reduce the costof such a control circuit, the second transistor that is activatedduring braking is a low-power transistor.

An object of the present invention is to propose means making itpossible to protect such a circuit from polarity reversal.

In the state of the art, the problem of a component being destroyed byaccidental polarity reversal has already been solved.

Reference can be made, in particular, to document U.S. Pat. No.5,519,557 in which the protection is implemented on the basis of a powerMOS transistor that is disposed in series with the device to beprotected.

Reference can also be made to document WO-A-00/24107 which also uses aprotection configuration implemented by using a power MOS transistor inseries.

Finally, reference can be made to document EP-1,045,501 in whichprotection of the device is provided by a power MOS transistor that isconnected in parallel with the load.

Essentially, such protection devices suffer from two drawbacks for whichthe invention provides remedies.

In the first drawback, failure of the protection device prevents theprotected device from operating even if the polarities applied areappropriate.

In the second drawback, since they are situated outside the protecteddevices, such protection devices always constitute additional componentsthat increase the cost of manufacturing and of assembling a device thatis protected from polarity reversal.

The present invention remedies those drawbacks of the state of the artin that it provides a control circuit for controlling a DC electricmotor from a biased DC voltage source, the control circuit being of thetype including at least one controllable interrupter connected to thehot spot of the electric motor, and a brake circuit serving to apply abraking short-circuit for braking the DC electric motor. The inventionis characterized in that the control circuit comprises a low-powercomponent that is connected in series in the brake circuit and that hashigh impedance in the event of polarity reversal and low impedance inthe event of correct polarity.

The invention also provides a vehicle windshield wiper systemincorporating such a control circuit.

Other advantages and characteristics of the present invention will bebetter understood with reference to the accompanying figures, in which:

FIGS. 1 to 7 are circuit diagrams explaining the state of the art andthe problems remedied by the present invention;

FIG. 8 is a circuit diagram of a first embodiment of the presentinvention;

FIG. 9 is a circuit diagram of a second embodiment of the presentinvention.

FIG. 1 shows an example of a conventional control circuit forcontrolling a DC electric motor that is more particularly for use in avehicle windshield wiper system.

The control circuit essentially comprises two metal oxide semiconductorfield effect (MOSFET) transistors, namely a first transistor M1 and asecond transistor M2, connected in series between a power supplypositive terminal 1 and the electrical ground generally constituted atthe pack of the vehicle.

The positive terminal of the battery or of the on-board network isconnected to the terminal 1 of the control circuit when said controlcircuit is in action.

The two MOS transistors are of the same type.

The source of the upper first transistor M1 is connected to the drain ofthe lower second transistor M2.

The drain of the upper first transistor M1 is connected directly to theterminal 1 of the control circuit while the common point between thefirst and the second transistors M1, M2 is connected to a first powersupply terminal of the DC motor M.

The other terminal of the DC motor M is connected to ground or to thepack of the vehicle.

Each gate 2 and 3 of the transistors M1 and M2 is connected in turn to adrive circuit (not shown) which makes it possible to decode the commandscoming from the driver station of the vehicle.

In a normal operating mode, an output terminal of the drive circuit isconnected to the gate terminal 2 of the upper transistor M1 and isactive when it is desired to switch on the motor M.

In this normal operating mode of the motor, a current i1 passes throughthe drain-source path of the first transistor M1, and then passesthrough the motor M to ground.

No current passes through the second transistor M2.

Conversely, another terminal of the drive circuit is connected to thegate 3 of the second transistor M2 and is activated when it is desiredto brake the motor M, e.g. to accompany the change of wiping directionof the wiper system.

In the braking operating mode, the current i1 is reduced to zero and acurrent i2 passes through the drain-source path of the second transistorM2 that is generated by the motor M. The time for which said current i2flows is short, and the energy in the transistor M2 is therefore lowerthan in the transistor M1, which explains that the transistor M2 can bedimensioned to be of lower power.

In reality, as is known, transistors of the MOSFET type, like thetransistors M1 and M2 have an inherent diode constituted between thedrain and the source connections. This anti-parallel diode of a MOSFETtransistor is created during manufacture of the transistor. Allpresent-day MOSFET transistors have a parasitic diode and those that donot have one are manufactured using a special method and are too costlyfor most uses.

FIG. 2 shows that configuration in detail.

In FIG. 2, a MOSFET transistor 4 is shown with its gate electrode 5, itsdrain electrode 6, and its source electrode 7.

In particular, the transistor shown is of the MOS type having anN-channel 7. An anti-parallel diode 8 is constituted during manufactureof the transistor 4 so that its anode is connected to the source of thetransistor 4 and so that its cathode is connected to the drain of thetransistor 4. In fact, relative to a command on the gate 5, said diode 8operates in parasitic manner. In particular, it tends to causeelectrical energy to pass along the source-drain path in the directionopposite to the normal flow of charge carriers in the absence of acommand on the gate.

In such a situation, the MOSFET transistor can easily be destroyed ifthe reverse current exceeds a certain threshold.

FIG. 3 shows the transistors M1 and M2 of the FIG. 1 circuit with theiranti-parallel diodes D1 and D2 when they are connected erroneously withreversed polarity.

The transistors M1 and M2 and the motor M are connected as in FIG. 1.

However, the positive and the ground terminals are reversed, the drainelectrode of the transistor M1 being connected to ground and the sourceelectrode of the transistor M2 being connected to the positive terminal.

As a result, as soon as the erroneous connection is made, the drivecircuit is also reverse biased and is thus not operational. Theanti-parallel diodes D1 and D2 of the transistors M1 and M2 conduct andthey pass a high current which is not limited by any load, and which istherefore destructive.

In general, it is the transistor M2 that is destroyed because it isdimensioned to be of a power lower than the power of the transistor M1,but the transistor M1 can also be destroyed if the transistor M2presents a short-circuit on being destroyed.

As a result, on its drain-source path, the transistor M2 sees a currentof direction opposite to the nominal direction of flow of the chargecarriers, and it is irremediably destroyed if, as is usual, it isdimensioned only for braking the rotor armature of the motor M.

FIG. 4 shows first polarity reversal protection means taken from theabove-mentioned state of the art.

The positive terminal of a biased power supply source is connected tothe anode of a protection diode 12 whose cathode is connected to theinput terminal or hot spot which is serves to be connected to thepositive power supply terminal of an electronic unit to be protected 13.

The other biasing terminal of the electrical circuit 13 is connected toground 14.

In this configuration, the protection diode 12 for protecting frompolarity reversal gives rise to a voltage drop that can be detrimentalto proper operation and, in the event that it is destroyed, interruptsthe power supply to the electronic unit.

FIG. 5 shows another embodiment of a protection system of the state ofthe art.

In the state of the art, the positive power supply terminal 11 istransmitted to the positive input terminal of the electronic unit 13 viathe drain-source of an N-channel MOSFET transistor that has a parasiticanti-parallel diode 16.

The gate of the MOSFET transistor 17 is connected to a command signal 15formulated by the electronic unit to be protected 13.

Such a device makes it possible to reduce the voltage drop in normaloperating mode by a command on the gate of the transistor 17.

However, in the event that the MOSFET transistor 17 is destroyed, theelectronic unit 13 loses its power supply, which can be detrimental.

In addition, the electronic unit 13 to be protected must also bemodified in order to generate a correct signal 15 for causing the MOSFETtransistor 17 to conduct.

FIG. 6 shows another polarity reversal protection mode in the state ofthe art.

A protection diode 18 is connected between the cold spot of theelectronic unit 13 to be protected and the electrical ground of thedevice.

This type of circuit suffers from two drawbacks caused firstly by thevoltage drop induced by the diode 18 in normal operation that places thecold spot of the power supply of the electronic unit 13 at a few voltsabove the electrical ground of the circuit.

The other drawback also results from the fact that, in the event of acurrent surge, the diode 18 can accidentally be destroyed, and theelectronic unit 13 is then no longer powered.

FIG. 7 shows another example of a state-of-the-art polarity reversalprotection system.

In that state of the art, a MOSFET transistor 21 is placed via itsdrain-source path in reverse between the cold spot of the electronicpower supply 13 and ground 14.

The anti-parallel diode 20 of the MOSFET transistor 19 plays the samerole as the diode 18 of the example shown in FIG. 6.

However, the voltage drop can be reduced by causing conduction to takeplace in normal operating mode via the gate electrode 22 of the MOSFETtransistor 21.

In order to remedy the above-mentioned drawbacks of the state of theart, and in order to offer novel advantages, provisions of the inventionare shown in a first embodiment in FIG. 8 and in a second embodiment inFIG. 9.

In FIGS. 8 and 9, the transistors M1 and M2 are substantially identicalto the transistors M1 and M2 of the circuit of FIGS. 1 and 3.

The control circuit of the invention comprises a low-power component M3connected in series in the braking circuit. This component has highimpedance in the event of polarity reversal of the power supply of thecontrol circuit and therefore of the motor, and low impedance in theevent of correct polarity.

In one embodiment of the invention, the component M3 is a dipole whichcan be caused to go to its low impedance state by at least one electrode26 connected to an armature braking control signal.

The gate electrode 23 of the high-power MOSFET-type transistor M1 iscontrolled directly and is placed in the active state when the motor Mis to be switched on.

The low-power second MOSFET transistor is connected between the commonpoint of connection between the electric motor and the source of thetransistor M1, and is, via its source, connected to a low-impedanceelectric dipole for protecting from polarity reversal.

The control circuit must not activate both transistors M1 and M2 at thesame time.

However, when the brake circuit is activated by a pulse on the gateelectrode 24 of the second transistor M2, the low-power component 25goes to or remains in the low-impedance state, and the braking currentgoes to ground via the dipole 25.

In the first embodiment of the invention shown in FIG. 8, the controlcircuit for controlling a DC electric motor is such that the low-powercomponent M3 is constituted by the anti-parallel diode of a MOSFETtransistor whose source is connected to the source of the transistor M2and to the anode of its anti-parallel diode, and whose drain isconnected to the cathode of the above-mentioned diode and to ground.

The gate electrode of the MOSFET transistor 25 is connected directly tothe control terminal of the brake circuit 24 so that, when the polarityis appropriate, and when the circuit is activated, the transistor 25passes the current coming from the transistor M2.

In the principle of the invention, the dipole 25 goes to or remains inthe low-impedance state. In the configuration of the first embodimentshown in FIG. 8, and in the event of polarity reversal, the transistorM3 has high impedance because, in this case, its anti-parallel diodecannot conduct, since its cathode is connected to the positive terminalof the power supply source, and the transistor M3 cannot be activatedbecause the drive circuit is reverse biased and is not operational.

In the second embodiment of the invention, shown in FIG. 9, in whichelements identical to elements in FIG. 8 have like references and arenot described in any more detail, the control circuit of a DC electricmotor is such that the low-power component M3 is constituted by a diodeconnected via its cathode to the normal ground of the power supply andwhose anode is connected to the source of the braking transistor M2.

In a second type of embodiment not shown in the drawing, the gate of thetransistor M3 is not necessarily connected to the gate of the transistorM2. In such an embodiment, the transistor M3 can be activated (on itsgate) by a permanent DC voltage coming from the drive circuit, such as,for example, the regulated voltage of the drive circuit, and thereforethe transistor M3 conducts continuously so long as the power supplypolarity is applied appropriately. It is the transistor M2 that allowsthe current to pass through the branch M2-M3. This type of configurationoffers the advantage over the type of the first embodiment shown in FIG.8 of not preventing the anti-parallel diode of the transistor M2 frombeing used as a recovery diode when the load is inductive, which applieswhen the load is constituted by an electric motor. In this type ofconfiguration of the second embodiment, it is thus necessary to separatethe controls for the gates of the transistors M2 and M3 of the firstembodiment shown in FIG. 8.

In the above-defined second type of embodiment, the gate electrode ofthe transistor 25 is not necessarily connected to the terminal 24. Thetransistor 25 can be activated by an activation signal connected to itsgate by a permanent DC voltage coming from the drive circuit. However,when the polarity is appropriate and when the circuit is activated, thetransistor 25 does indeed pass the current coming from the transistorM2.

Conversely, when the control system is subjected to polarity reversal,the anti-parallel diode of the transistor M1 conducts.

As a result, the anti-parallel diode of the transistor M3 preventscurrent from passing, thereby preventing the circuit from operating.

In the event that the transistor M3 is accidentally destroyed, only thebraking function is interrupted.

In particular, since the gate electrodes of the transistors M2 and M3are connected to the same external control terminal for connection to adrive circuit (not shown) of the control circuit of the invention, it isvery easy to mount the two transistors M2 and M3 on a common support,e.g. on a brake circuit support.

Such mounting on a common support can be used even if the gates of thetransistors M2 and M3 are not connected together as in the secondembodiment described above. Nevertheless, it is possible to use packagesincorporating two transistors of the same size in order to save space onthe support, e.g. a printed circuit. The gates of each transistor remainaccessible separately, which makes it possible to apply differentvoltages to each of them.

As is known, a transistor M2 can be mounted in a half-bridgeconfiguration with the high-power transistor M1 on a single, commonsupport such as a printed circuit with heat sinks or an integratedcircuit.

In the invention, it is then possible to mount the transistors M1, M2and the polarity reversal protection MOSFET transistor together on asingle, common support such as a printed circuit and/or an integratedcircuit.

A wiper system of the invention can incorporate a half-bridge of thetype shown in FIG. 8 and which is connected directly to a wiper electricmotor.

A drive circuit is added in order to connect the gate electrodes of thetransistors M2 and M3 of the brake circuit to a braking controlterminal, and the gate electrode of the transistor M1 is connected to acontrol terminal for switching on the electric motor either by means ofa pulse command or else by means of a periodic command voltage fordriving the electric motor in voltage-regulated manner with pulses ofwidth controlled by the drive circuit as a function of setpoints (pulsewidth modulation (PWM) mode).

It should be noted that in the second type of embodiment and not shownin the drawings, the gates of the transistors M2 and M3 are notnecessarily connected together, but rather they can be connected tosuitable control terminals of their control circuit.

In one embodiment, the control circuit generates a pulse command or aPWM command using the configuration of FIG. 8. A thyristor can be usedfor a pulse command, and it replaces the transistors M2 and M3. But theabsence of recovery diode then constitutes a drawback.

1. A control circuit for controlling a DC electric motor from a biasedDC voltage source, the control circuit being of the type comprising atleast one controllable interrupter (M1) connected to the hot spot of theelectric motor, and a brake circuit (M2) serving to apply a brakingshort-circuit for braking the DC electric motor, said control circuitbeing characterized in that it comprises a low-power component (M3) thatis connected in series in the brake circuit and that has high impedancein the event of polarity reversal and low impedance in the event ofcorrect polarity.
 2. A control circuit for controlling a DC electricmotor according to claim 1, said control circuit being characterized inthat the component (M3) is a dipole which can be caused to go to itslow-impedance state by at least one electrode (26) connected to anarmature braking control signal.
 3. A control circuit for controlling aDC electric motor according to claim 1, said control circuit beingcharacterized in that the low-power component (M3) comprises a diodeconnected via its cathode to the normal ground of the power supply.
 4. Acontrol circuit for controlling a DC electric motor according to claim3, said control circuit being characterized in that the diode isconstituted by the anti-parallel diode of a MOSFET transistor (M3).
 5. Acontrol circuit for controlling a DC electric motor according to claim2, said control circuit being characterized in that the low-powercomponent comprises a thyristor optionally supplemented by a recoverydiode.
 6. A control circuit for controlling a DC electric motoraccording to claim 1, said control circuit being characterized in thatthe low-power component (M3) is integrated on the same support as thebrake circuit (M2).
 7. A control circuit for controlling a DC electricmotor according to claim 1, said control circuit being characterized inthat the low-power component (M3) is integrated on the same support asthe control circuit (M1, M2).
 8. A vehicle windshield wiper system,characterized in that it comprises at least one electric motor and/or atleast one control circuit (M1-M3) according to any preceding claim, anda drive circuit for placing the control circuit in an ON state or in abraking state.
 9. A wiper system according to claim 8, characterized inthat the drive circuit comprises means for activating the low-powercomponent (M3) of the control circuit by means of a pulse command or bymeans of a pulse width modulation (PWM) command.
 10. A wiper systemaccording to claim 8, characterized in that the drive system comprisesmeans for activating the brake circuit of the control circuit by a PWMcommand, and in that the low-power component (M3) is maintained activeby a separate voltage command, such as the regulated power supplyvoltage of the drive circuit, when the power supply polarity of thesystem is correct.